100amp Main breaker warm to the touch

I run a 15HP RPC which is connected to a 60Amp breaker. I can run it all day and the motor barely gets warm and the 60 Amp is never warmer than room temperature. Yesterday I had the RPC running, my CNC lathe running, my 5hp air compressor running nearly constantly, and a window air conditioner running and it tripped the 100Amp main. I shut everything off and when I went to turn the breaker back on it was very warm to the touch. So I figured I just had too much stuff running at once. I left the window air conditioner off, shut off the air compressor and finished my turning project.

This morning I started everything up - except the window air conditioner and everyting has been running for about two hours but I went over and touched the 100Amp main and it is warm to the touch again. I don't know if it has always been warm when I am running everything or not. Could the trip yesterday have weakened the breaker and now I need to replace it? The circuit breaker is probably about 15 years old and I have been running RPC's in here all that time. The thing that perplexes me is that the 60Amp that the RPC is connected to is not even warm and it seems like that one should be the one that would give me trouble if any. Maybe I am just being paranoid now. Any ideas?

Check all the wire connections into and out of the breaker to make sure that the full number of strands actually are in the terminal and tightly clamped; sometimes when people strip wire ends they strip off a few strands of copper also.

I have had breakers get warm and trip when they were certainly not running at full rating and this is what has caused it, a high resistance connection on one of the lines. Sometimes the terminal and wire were discolored and the insulation distorted from high localized heat.

I disconnected the breaker and removed it. I looked it over and do not see any problems. I put it back and made sure the wires were tight. I turned the power back on and started up several things and it is getting warm again. What next?

Breakers do 'wear out', but usually after tripping many times. Make sure the prongs aren't soft and annealed where the breaker attaches to the buss. Heat will make them lose their temper which makes them lose there grip...which equates to a high resistance junction and heat.

Breakers are thermal devices, so they need to heat to operate properly. If the load is close to the breaker rating it will become warm. The least I would do is stick an Amprobe on one leg and see exactly what the average load is.

Breakers are pretty cheap so replacing it wouldn't be out of the question. I'm not an electrician, but this is how I see it.

I disconnected the breaker and removed it. I looked it over and do not see any problems. I put it back and made sure the wires were tight. I turned the power back on and started up several things and it is getting warm again. What next?

Replace it.

Possibly if you have access to an AC ammeter you should check what your total load is. Strictly speaking the maximum continuous current through a 100 amp breaker should be limited to something like 85amps. You may be faced with upgrading the breaker and some wiring if you cannot transfer some loads to another supply.

I do not have an amprobe/ammeter. I did switch off everything except the lights and the breaker cooled down. I waited about an hour and started the RPC with no machinery running and the breaker got warm again. I left it running about 10 minutes and it seemed to fluctuate between very warm and just warm. But what I don't understand is why the main 100 Amp breaker is getting warm with the RPC running but the 60 Amp breaker that the RPC is connected to is not warm at all. Suggestions?

I think Mud has it; if it is not getting warm because there is a bad connection outside then possibly it is something inside so it is probably time to install a new breaker. They do not last forever and it is better to be safe than sorry.

Ok, it seems as though the front bearing may be going on my rpc motor because when I touch the housing in that area it is pretty hot after running a while. Could that be causing the breaker to get warm? - but again - why isn't the 60 amp breaker that the RPC is connected to getting warm? - only the 100 amp main is getting warm.

Joe,
When you go shopping for replacements, keep in mind there are some counterfeit devices out there. Square D, for example, has a section on their website showing what to watch for.

Searching out any high-resistance connections is worth doing. As a non-electrician, I had to 'provoke' one to fail back in the day... one exciting fireworks display replaced the slow melting of lots of wires in lots of conduits. It's better to deal with it when you decide, rather than when the breaker decides...

Chip,
Can you tell me why only the 100 amp main is getting warm and not the 60 amp breaker that the RPC is connected to. This is what has me most puzzled. The 100 amp main feeds the panel then the 60 amp breaker feeds the RPC. The 100 amp main only gets hot when the RPC is running. So the current is flowing through the 100 amp main to the 60 amp circuit that feeds the RPC - since the 100 amp main only gets hot when the RPC is running, why isn't the 60 amp breaker getting hot also? What am I not understanding here?

A toaster uses ni-chrome wire to offer resistance, heat up cherry red and burn your toast. If current is passing through a dirty or loose connection on or in your 100 amp breaker, it is acting just like the little coils in the toaster. The funky connection is offering a high resistance to current flow which causes the connection and surrounding material to heat up.

If your RPC is drawing the exact same current through the 100 amp breaker as it is through the 60 amp breaker (which it is) but the 100 amp breaker is heating up, it is mechanical problem with that breaker...a high resistance connection, either with the buss bars or a internal glitch. That's how I see it.

Your breaker is showing signs of failure. The contacts on the inside have more resistance than normal so the current flow is creating heat. If you checked the resistance of the 100 amp breaker and compared it to the 60 amp breaker you would find that the 100amp breaker is showing considerably more (fraction of an ohm versus probably several ohms.

power= current*current* resistance

Do you understand now that a slight increase in resistance has a large effect on power being converted to waste heat inside the breaker.

Healthy breakers running at half load should not exhibit any appreciable heat.

That's why we run thermal scans on switch gear, starters, and large panels. Heat is one of the best early signs for detecting impending failure.

Thanks Stuart. That's what I am trying to get to the bottom of. If I touch the wires that feed the 100 amp main 1/2" away from where they go into the lugs on the breaker, they are no more than room temperature. I have checked the connection of the wires to the breaker three times and they are tight for sure. So at this point I am guessing it is a mechanical failure with the breaker but I don't have much experience with breakers to know that for sure and I was trying to get some insight in one direction or the other before I just start replacing stuff or tearing stuff apart.

Your breaker is showing signs of failure. The contacts on the inside have more resistance than normal so the current flow is creating heat. If you checked the resistance of the 100 amp breaker and compared it to the 60 amp breaker you would find that the 100amp breaker is showing considerably more (fraction of an ohm versus probably several ohms.

power= current*current* resistance

Do you understand now that a slight increase in resistance has a large effect on power being converted to waste heat inside the breaker.

Healthy breakers running at half load should not exhibit any appreciable heat.

That's why we run thermal scans on switch gear, starters, and large panels. Heat is one of the best early signs for detecting impending failure.

Thanks Ziggy. It is getting clearer and clearer in my mind. I was just afraid to jump to the conclusion that my 100 amp main breaker is bad if there is some other problem that is contributing to it. My buddy is coming over tomorrow afternoon with an amprobe so we can make double sure that everything is within safe operating specs.

One of the problems I have is in finding a local electrician to help me with my wiring because as soon as you throw an RPC in the mix it is out of the realm of the residential guys who normally only deal with 110/220 v single phase or the commercial guys who normally only deal with 440 v 3 phase. I have found much more knowledgable guys on this board who may not be licensed electricians but they have much more thorough electrical knowledge.

That would mean you are using the 1" wide style snap in breakers such as the Homeline series from Square D ?

Those types of breakers are notorious for doing just what you are describing mainly due to their lack of good contact with the bus strips and that some cheaper panels had aluminum bus in them. They hardly make a decent residential panel let alone one being used near or at its max rating. I have seen many a panel with burnt bus strips. Basically an electrical fire waiting to happen.

I know it costs money but an upgrade to electrical equipment sounds to be in order. I would recommend no less than a Square D QO panel and for your application and with having a high load on certain breakers the QO D series panels provide for both snap in and bolt in breakers. Square D QO breakers are also BOTH thermal (overload) and magnetic (short circuit) rated breakers.

I do have some used panels but the only smaller 120/208 I have right now is a Cutler-Hammer 200 amp outdoor rated panel(it would be my second choice behind Sq D). Unless you wanted to step up to 480 volts then I have some whoppers.

Based on how you've responded to the guestions, I tend to agree with Ziggy1. I would change the 100 amp main breaker.
You may consider going up to a 150 main , BUT ONLY IF THE INCOMING WIRE CAN SUPPORT THE INCREASED AMPS! I did this at my house, I went from 100 up to 200 amps. Had a lic. electriction do it and all he did was change the main panel and breaker. I called the power company first, and they said the wire coming in was more than enough to handle the upgrade in amps. They didn't even come out to check the job because I used the contractor. He did it as a side job for 175 bucks. I paid around $160 for the parts as I recall.
Steve

Advice.

Tim- Joe's problem is localized to the 100A MAIN breaker, which is a Siemens... so the HOMline (crap) issue isn't what he's dealing with (but I agree, HOMlines are junk).

Joe- Aside from a problem with the breaker, there are two other things that MAY contribute to the problem, I believe you've already witnessed proof that these other possibilities are not the cause... local heating from bad connection, or magnetic problem. Since the breaker is hot, it's generating the heat. Typically, when you hook up a breaker, you'll connect copper or aluminum wire to the terminals. When connecting, the breaker manufacturer will indicate wether aluminum is acceptable for connection, and if so, should some sort of conductive grease be used to protect and 'buffer' the joint. SINCE the wire is cold, safe to assume that thermal percolation is NOT originating at the joint, but rather occuring INSIDE the breaker. Connection of main breaker to the busbar may also be the problem, but without shooting the temp of the busbar, it'd be hard to say. I use a laser-guided thermometer to look at those things... keeps my heart from being fibrillated at 60hz.

One OTHER thing that MAY be messing it up a bit... if there's enough reactive power going backwards from the RPC through the panel, it may be converting some of that reactive power into heat... but it'd be the first time I ever saw something like that in a 3-phase panel.

Change the main breaker. Use conductive grease or anti-oxidant as perscribed by your electrical physician. Avoid cranberries, don't operate heavy machinery or play poker for at least six hours after taking advice.

DK :-)

Originally Posted by alcosteam

That would mean you are using the 1" wide style snap in breakers such as the Homeline series from Square D ?

Those types of breakers are notorious for doing just what you are describing mainly due to their lack of good contact with the bus strips and that some cheaper panels had aluminum bus in them.

Posting Permissions

ADVERTISEMENTS

ABOUT Practical Machinist

With more than 10.6 million unique visitors over the last year, Practical Machinist is the most visited site for metalworking professionals. Practical Machinist is the easiest way to learn new techniques, get answers quickly and discuss common challenges with your peers. Register for the world’s largest manufacturing technology forum for free today to stay in the know.